Pay-channel transmission system for CATV

ABSTRACT

A pay-channel transmission system for a CATV has an optical transmission unit and an optical receiver unit. The optical transmission unit includes pay-channel television signals optically multiplexed by a λ 1  wavelength semiconductor laser and non-pay-channel television signals optically multiplexed by a λ 2  wavelength semiconductor laser. The optical receiver unit includes an optical filter for filtering the λ 1  wavelength data. Thus, a non-subscriber having the optical filter is prevented from receiving the pay-channel television signals, and a subscriber having no optical filter is able to receive both the pay-channel television signals and non-pay-channel television signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a CATV pay-channel transmission methodwhich distributes paid-subscriber television signals to subscribers.

2. Description of the Prior Art

In conventional CATV systems using a standard coaxial cable transmissionpath unauthorized reception of pay-channel programming is prevented bythe transmitter scrambling out-going pay-channel television signals. Asignal receiver is provided to CATV subscribers to descramble thereceived signal using various methods, including a "key" which unlocksthe scrambling format or an electrical filter which passes only thepaid-for channels. Such methods have been described in "CATV in thesatellite broadcasting age", p. 252, (1987), compiled by CableBroadcasting Section, Broadcasting Administration Department, (Japanese)Ministry of Posts and Telecommunications.

However, there has always been the possibility of unauthorized receptionof pay-channel programming enabled by tampering with the electricalcircuits of the signal receiver. Picture deterioration in thedescrambled, demodulated signal has also been a problem. On the otherhand, unauthorized reception of digitally processed scrambled signals isdifficult and there is no degradation of the demodulated signal, but thedescrambler cost is higher due to the need for A/D and D/A converters.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide apay-channel transmission method whereby unauthorized reception isdifficult and there is no picture deterioration.

To achieve the aforementioned object, a pay-channel transmission methodaccording to the present invention comprises at the transmitting side afirst optical transmitter which transmits a pay-channel televisionsignal by means of a λ₁ wavelength semiconductor laser, and a secondoptical transmitter which transmits other television signals by means ofa λ₂ wavelength semiconductor laser, and comprises at the optical signalreceiving side one optical signal receiver and an optical means to blockthe λ₁ wavelength, thereby blocking the pay-channel optical signal.

Alternatively, two semiconductor lasers with differing bandwidths may beused to transmit the pay-channel television signal by means of a firstsemiconductor laser while wavelength multiplexing another televisionsignal by means of a second semiconductor laser, and an optical dividerwhich can be inserted and removed may be provided in the optical signalreceiver.

In addition, the transmitter may comprise a first optical transmitter tofrequency multiplex pay-channel television signals in the frequencyrange f₁ -f_(k) and non-pay-channels in a frequency range other than f₁-f_(k) for optical transmission by means of a λ₁ wavelengthsemiconductor laser, and a second optical transmitter which opticallytransmits an FDM signal, which is a carrier wave frequency multiplexedto the same f₁ -f_(k) frequency range, by means of a λ₂ wavelengthsemiconductor laser, said λ₂ wavelength being in the same wavelengthband as the λ₁ wavelength, and the receiver may comprise an opticalsignal receiver and an optical means to block the wavelength λ₂.

It is thus possible to block the wavelength carrying the pay-channelprogramming and thus interfere with the pay channel, thereby preventingunauthorized reception.

The present invention differs completely from conventional scramblingmethods used to process an electrical signal, and unauthorized receptionenabled by tampering with the electrical circuitry of the receiver isdifficult because interference is applied at the optical signal stage bymeans of such optical components as optical dividers and interferencefilters. Furthermore, because optical interference may be combined withconventional electrical signal scrambling, protection againstunauthorized reception is doubled.

In addition, distribution of deterioration-free video signals topay-channel subscribers can be accomplished quickly and economically byinserting or removing an optical component, thereby providing greaterpractical effectiveness.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given below and the accompanying diagrams wherein:

FIG. 1, FIG. 2, FIG. 4, and FIG. 5 are block diagrams of the preferredembodiments of the present invention, and

FIG. 3 is a diagram illustrating the wavelength characteristics of anoptical receiver element.

DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred embodiments of the present invention are describedhereinbelow with reference to the accompanying figures, whereof FIG. 1is a block diagram of a first embodiment. As shown in FIG. 1, thisembodiment comprises an (electro-optical) optical transmitter 1comprising a 1.3 μm wavelength semiconductor laser, an (electro-optical)optical transmitter 2 comprising a 1.5 μm wavelength semiconductorlaser, an optical combiner 3, a single mode optical fiber fortransmission 4, an interference filter 5, an (opto-electric) opticalreceiver 6, and a tuner 7.

The interference filter 5 blocks the 1.3 μm wavelength. The opticalelement used in the optical receiver 6 is a Ge element with uniformsensitivity in the 1.3-1.5 μm wavelength band.

As indicated by number 8 in FIG. 1, the pay-channel television signalsare amplitude modulated, frequency multiplexed with a carrier wave inthe band f₁ -f_(k) and converted to a 1.3 μm wavelength optical signalby the optical transmitter 1, and then sent to the Wavelength DivisionMultiplexing (WDM) coupler 3. Non-pay-channel television signals areamplitude modulated, frequency multiplexed with a carrier wave in theband f_(k+1) -f_(k+j+1) and converted to a 1.5 μm wavelength opticalsignal by the other optical transmitter 2, and then sent to the WDMcoupler 3.

The optical signal combined by the WDM coupler 3 is transmitted via theoptical fiber transmission cable 4 to the subscriber receiver and inputto the interference filter 5 thereof. The 1.5 μm wavelength opticalsignal which can pass through the interference filter 5 is thenconverted to an electrical signal by the optical receiver 6. Thesubscriber can thus tune in and view programs which are not carried on apay-channel, but cannot view any pay-channel transmissions.

When the subscriber contracts for pay-channel programming, both the 1.3μm and 1.5 μm wavelength optical signals can be input to the opticalreceiver 6 by simply removing the interference filter 5, and allchannels transmitted from the CATV broadcast center can thus be viewed.This is dependent upon the wavelength sensitivity of the opticalreceiver element having an approximately constant wavelength band andthe frequency assignments of pay-channel and non-pay-channel programmingbeing different. Because the pay-channel television signals are blockedby the interference filter 5 in this embodiment, there is theoreticallyno deterioration of the signal quality when the scrambled electricalsignal is descrambled on the receiving side.

An alternative embodiment of the present invention is describedhereinbelow with reference to the block diagram in FIG. 2. It is to benoted that those components which are functionally equivalent tocomponents in FIG. 1 are indicated with like reference numbers andfurther detailed description thereof is omitted in the following.

As shown in FIG. 2, this embodiment comprises an (electro-optical)optical transmitter 1 which transmits a frequency multiplexedpay-channel television signal (FDM signal) by means of a long (e.g., 1.3μm) wavelength band semiconductor laser, an (electro-optical) opticaltransmitter 2 comprising a short wavelength (e.g., 0.8 μm) bandsemiconductor laser which transmits an FDM signal frequency multiplexingthe other channels, a WDM coupler 3 which combines the short and longwavelength band signals, an optical divider 51 which divides the shortand long wavelength band optical signals, an (opto-electric) opticalreceiver 61 using a Ge optical receiver element for the long wavelengthband, and an (opto-electric) optical receiver 62 using an Si opticalreceiver element for the short wavelength band. FIG. 3 is a diagramillustrating the wavelength sensitivity characteristics of the opticalreceiver elements (This diagram is introduced in Introduction to opticalfiber communications, Suematsu, Yasuhara, et al., p. 104; The OHM-ShaLtd.)

The pay-channel FDM signal is input to the optical transmitter 1,converted to a 1.3 μm wavelength optical signal, and output to theoptical combiner 3. The FDM signal for the other channels is similarlyinput to the optical transmitter 2, converted to a 0.8 μm wavelengthoptical signal, and output to the WDM coupler 3. The WDM coupler 3combines the 0.8 μm and 1.3 μm signals and transmits the combined signalvia the optical fiber transmission cable 4. The broadcast signal isinput to the optical divider 51, which divides the signal into long andshort wavelength band optical signals, which are respectively receivedby the long wavelength band optical receiver 61 and the short wavelengthband optical receiver 62 and converted to electrical signals.

If the subscriber has not contracted for pay-channel service, theoptical divider 51 is removed and the optical fiber transmission cable 4is connected directly to the short wavelength optical receiver 62. Boththe long and short wavelength optical signals are thus input to theoptical receiver 62, but because the sensitivity of the Si elementtherein is extremely low to the long wavelength band as shown in FIG. 3,the long wavelength band optical signal cannot be received. Thus, thenon-subscriber can be prevented from viewing pay-channel programming.

FIG. 4 is a block diagram of an alternative embodiment of the presentinvention. It is to be noted that those components which arefunctionally equivalent to components in FIG. 1 and FIG. 2 are indicatedwith like reference numbers and further detailed description thereof isomitted in the following.

As shown in FIG. 4, this embodiment comprises (electro-optical) opticaltransmitters 1 and 2, a WDM coupler 3, an optical fiber transmissioncable 4, an interference filter 52, and an (opto-electric) opticalreceiver 6.

As indicated by reference number 10 in FIG. 4, the FDM signal, which isthe pay-channel television signals frequency multiplexed to the f₁-f_(k) frequency band and frequency multiplexed with the othertelevision signals to the f_(k+1) -f_(k+j+1) frequency band, is input tothe optical transmitter 1, converted to a 1.3 μm wavelength opticalsignal, and then output to the WDM coupler 3. The FDM signal 11, whichis an unmodulated carrier wave frequency multiplexed to the f₁ -f_(k)frequency range, is input as an interference signal to the other opticaltransmitter 2, converted to a 1.5 μm wavelength optical signal, andoutput to the optical combiner 3.

The optical signal generated by the WDM coupler 3 is transmitted to theoptical receiver by the optical fiber transmission cable 4, theinterference filter 52 of which passes only the 1.3 μm optical signal tothe optical receiver 6 for conversion to an electrical signal.

The interference filter 52 is inserted to the optical receiver ofpay-channel subscribers, thus enabling viewing of all channels broadcastfrom the CATV center. The interference filter 52 is not provided in theoptical receivers of non-subscribers to pay-channel service, and theoptical fiber transmission cable 4 is therefore connected directly tothe optical receiver 6.

When the interference filter 52 is not present, the optical signaloutput by the optical transmitter 2 is also input to the opticalreceiver 6. If a Ge element is used for the optical element in thereceiver, sensitivity is constant to optical signals in the 1.3 μm-1.5μm wave band, and the optical signals of both wavelengths are thusconverted to an electrical signal. Both FDM signals 11 and 10 are thusmixed in this electrical signal. Because the unmodulated carrier wave inthe interference signal FDM 11 occupies the same frequency position asthe pay-channel in the other FDM signal 10, the image quality ofpay-channel programs deteriorates.

An alternative version of this embodiment is shown in the block diagramof FIG. 5, wherein reference numbers 31, 32, and 33 are AM modulators,100 is an inversion amplifier, and 411 and 412 are mixers.

The video signals for the pay channels are input from input terminals311 to the AM modulator 31 and output to the mixer 411. The videosignals for the other channels are input from input terminals 322 to theAM modulator 32 and output to the mixer 411. The mixer 411 outputs thefrequency multiplexed AM-FDM signal to the 1.3 μm optical transmitter 1.

The video signals for the pay channels are simultaneously input to theinversion amplifiers 100, modulated by an AM modulator 33, and output tothe mixer 412 for frequency multiplexing. The mixer 412 then outputs themultiplexed signal to the 1.5 μm optical transmitter 2. It is to benoted that the frequency assignments of the carrier wave in the firstand last AM modulators 31 and 33 are the same.

The optical signals thus obtained from the optical transmitters 1 and 2are mixed by the WDM coupler 3 and transmitted over the optical fibertransmission cable 4.

The interference filter 52 is inserted to the optical receiver ofpay-channel subscribers, passing only the 1.3 μm wavelength opticalsignal and removing the 1.5 μm interference optical signal, and thusenabling viewing of all channels broadcast from the CATV center. Becausethe interference filter 52 is not provided in the optical receivers ofnon-subscribers to pay-channel service, setting the tuner 7 to apay-channel position causes demodulation of the video signal in whichthe optical signals from both optical transmitters 1 and 2 are combined.

While the video signals obtained from the two optical receivers are thesame, the polarities thereof are inverted, thus resulting in anirregular synchronizing signal in the mixed video signals. As a result,the channel cannot be viewed.

While an AM-FDM signal was used by way of example in the abovedescription, the same effect can be obtained by modulating the videosignal with another modulation method (e.g., FM) before transmission. Inaddition, the same effect can be obtained whether applied to a videosignal, digital signal, PSK or FSK modulated optical transmission.

As will be known from the above description, unauthorized reception ofpay-channel broadcasts can be effectively prevented by simply removingthe interference filter 52. Furthermore, pay-channel programming can bebroadcast without deteriorating the picture quality of the pay-channelprogramming when the interference filter 52 is inserted.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A pay-channel transmission system for a CATVcomprising:(I) an optical transmission unit comprising:a first opticalsignal generator for generating a frequency multiplexed pay-channeltelevision signal which is multiplexed to an f₁ -f_(k) frequency bandand outputs a first optical signal by means of a λ₁ wavelengthsemiconductor laser; a second optical signal generator for generating afrequency multiplexed non-pay-channel television signal which ismultiplexed to a frequency band other than f₁ -f_(k) and outputs asecond optical signal by means of a λ₂ wavelength semiconductor laser;and an optical combining means for combining said first and secondoptical signals; and (II) an optical receiver unit for receiving theoptical signal transmitted from the optical transmission unitcomprising:an optical blocking means for blocking said first opticalsignal and for transmitting only said second optical signal, whereby anon-subscriber having said optical blocking means is prevented fromreceiving the first optical signal, and a subscriber having no opticalblocking means is able to receive both the first and second opticalsignals.
 2. A pay-channel transmission system for a CATV comprising:(I)an optical transmission unit comprising:a first optical signal generatorfor generating a frequency multiplexed pay-channel television signalwhich is multiplexed to a first frequency band and outputs a firstoptical signal at a first frequency range by a first semiconductorlaser; a second optical signal generator for generating a frequencymultiplexed non-pay-channel television signal which is multiplexed to asecond frequency band and outputs a second optical signal at a secondfrequency range by a second semiconductor laser; and an opticalcombining means for combining said first and second optical signals; and(II) an optical receiver unit for receiving the optical signaltransmitted from the optical transmission unit comprising:an opticaldivider means for separating said first and second optical signals; andfirst converting means sensitive to said first frequency range forconverting said first optical signal to a first electric signal; secondconverting means sensitive to said second frequency range for convertingsaid second optical signal to a second electric signal; whereby anon-subscriber having no optical divider means and first convertingmeans is prevented from receiving the first electric signal, and asubscriber having said optical divider means and first converting means,as well as the second converting means, is able to receive both thefirst and second electric signal.
 3. A pay-channel transmission systemfor a CATV comprising:(I) an optical transmission unit comprising:afirst optical signal generator for generating an optically modulatedtelevision signal modulated by a λ₁ wavelength semiconductor, saidtelevision signal being a combination of pay-channel television signaland non-pay-channel television signal which are separatably added toeach other; a second optical signal generator for generating anoptically modulated interference signal modulated by a λ₂ wavelengthsemiconductor, said interference signal having a characteristic tointerfere with said pay-television signal; an optical combining meansfor combining said optically modulated television signal and saidoptically modulated interference signal; and (II) an optical receiverunit for receiving the optical signal transmitted from the opticaltransmission unit comprising:an optical blocking means for blocking saidoptically modulated interference signal and for transmitting only saidoptically modulated television signal, whereby a non-subscriber havingno blocking means receives both the optically modulated interferencesignal and the optically modulated television signal and thus isprevented from receiving the pay-channel television signal, and asubscriber having said blocking means is able to receive the pay-channeltelevision signal, as well as the non-pay-channel television signal. 4.A pay-channel transmission system for a CATV comprising:(I) an opticaltransmission unit comprising:a first optical signal generator forgenerating an optically modulated television signal modulated by a λ₁wavelength semiconductor, said television signal being a combination ofpay-channel television signal which is frequency multiplexed to an f₁-f_(k) frequency band and non-pay-channel television signal which isfrequency multiplexed to a frequency band other than f₁ -f_(k), a secondoptical signal generator for generating an optically modulatedinterference signal modulated by a λ₂ wavelength semiconductor, saidinterference signal being frequency multiplexed to the f₁ -k_(k)frequency band, an optical combining means for combining said opticallymodulated television signal and said optically modulated interferencesignal; and (II) an optical receiver unit for receiving the opticalsignal transmitted from the optical transmission unit comprising:anoptical blocking means for blocking said optically modulatedinterference signal and for transmitting only said optically modulatedtelevision signal, whereby a non-subscriber having no blocking meansreceives both the optically modulated interference signal and theoptically modulated television signal and thus is prevented fromreceiving the pay-channel television signal, and a subscriber havingsaid blocking means is able to receive the pay-channel televisionsignal, as well as the non-pay-channel television signal.
 5. Apay-channel transmission system for a CATV comprising:(I) an opticaltransmission unit comprising:a first optical signal generator forgenerating an optically modulated television signal modulated by a λ₁wavelength semiconductor, said television signal being a combination ofpay-channel television signal and non-pay-channel television signal bothof which are modulated by a predetermined modulation system; a secondoptical signal generator for generating an optically modulatedinterference signal modulated by a λ₂ wavelength semiconductor, saidinterference signal being an inverse of said pay-channel televisionsignal which is modulated by said predetermined modulation system; anoptical combining means for combining said optically modulatedtelevision signal and said optically modulated interference signal; and(II) an optical receiver unit for receiving the optical signaltransmitted from the optical transmission unit comprising:an opticalblocking means for blocking said optically modulated interference signaland for transmitting only said optically modulated television signal,whereby a non-subscriber having no blocking means receives both theoptically modulated interference signal and the optically modulatedtelevision signal and thus is prevented from receiving the pay-channeltelevision signal, and a subscriber having said blocking means is ableto receive the pay-channel television signal, as well as thenon-pay-channel television signal.